Recently, accompanying the advance of silicon LSI technology, solid state imaging elements in which a plurality of photodetectors are arranged in a two-dimensional array on a semiconductor substrate and connected with a charge sweep device (hereinafter referred to as CSD) or a charge coupled device (hereinafter referred to as CCD) have been developed and put into practical use. Schottky-barrier diodes or photodiodes utilizing p-n junctions are usually employed for the photodetectors. Such solid state imaging elements are called "infrared imaging elements" or "visible light imaging elements" depending on the wavelength to be detected.
FIG. 12 is a block diagram of an infrared CSD imaging device in which a plurality of infrared detectors are arranged on a silicon substrate and scanning is carried out using CSD, disclosed in pages 42 to 48 of Defense Technology Journal, Vol.8, No.8, August 1987, published by Defense Technology Foundation. This device is constituted by a camera head 30, a signal processing part 31 and a monitor TV 33. In addition, this device employs a 512.times.512-element two-dimensional array type IRCSD (Infrared CSD) 32 as the imaging element. Since this IRCSD 32 scans electrically, a mechanical scanner is dispensed with, resulting in a small sized and light-weight camera head 30. In addition, since the camera head 30 includes a Stirling cycle refrigerator 302 utilizing a closed cycle for cooling the infrared detectors down to 77K, it is not necessary to provide a cooler.
An interline transfer CCD (hereinafter referred to as IL-CCD) is generally used for the charge transfer part of the two-dimensional element. In this IL-CCD, as shown in FIG. 13, one potential well for vertical transfer is provided for one detector and signal charges are transferred in the vertical and horizontal directions by a so-called bucket brigading system. Although this IL-CCD has very low noise, there is a limitation in its charge transfer ability. More specifically, when the signal charge amount increases and signal charges from one pixel exceed the storage capacity of one bucket, the signal charges are mixed with those from another pixel. In order to avoid this, it is necessary to increase the dimension of the vertical CCD. However, when the dimension of vertical CCD is increased, the fill factor (the ratio of the photodetector area to the pixel size) decreases, resulting in a reduction in sensitivity.
As another charge transfer system, there is a MOS system utilizing a MOS switch for reading out signals. The MOS system has an advantage over other systems in having a larger saturation charge amount. However, it has a disadvantage in that a large noise arises due to a large signal line capacitance and fixed pattern noise arises due to variations in the characteristics of the MOS switch. Although a reduction in pixel size is required for miniaturization and high resolution, a reduction in the pixel size induces a reduction in the signal charge amount obtained from one pixel. Thus, noise is a serious problem in the MOS system.
On the other hand, a CSD (Charge Sweep Device) used in the device of FIG. 12 is a new vertical charge transfer element, which has a large saturation charge amount and a noise level as high as that of the IL-CCD.
A description is given of the operation of the CSD with reference to FIGS. 13 and 14, which are shown in pages 41 to 45 of a journal "Television Technology" of September 1985.
As shown in FIG. 13, the transfer gates a CSD are controlled separately from each other. Only a transfer gate is selected in a vertical row during a horizontal period. In FIG. 13, only the second transfer gate from the left is turned on and signal charges in the photodiode connected to the transfer gate are transferred to the CSD. Other photodiodes are accumulating signal charges at this time.
The above operation will be described in detail with reference to FIG. 14. In the CSD, the signal charges are transferred by the charge sweep-out operation, in which the potential wall pushes the signal charges to the horizontal CCD as shown in FIGS. 14(b) to 14(d). A storage gate is provided between the horizontal CCD and the CSD, and the swept signal charges are stored in the storage gate as shown in FIG. 14(e). The sweep-out operation is completed in a horizontal period and the signal charges stored in the storage gate are transferred to the horizontal CCD during a horizontal blanking period as shown in FIG. 14(f) and then they are successively read out.
As described above, in the CSD, one vertical transfer element forms one potential well and the signal charges from one photodiode are output to the potential well. Therefore, sufficient signal charges can be obtained even when the channel width is reduced.
FIG. 9 shows a structure of a conventional infrared solid-state imaging element including Schottky barrier diodes serving as photodetectors, CSDs serving as vertical charge transfer circuits and a CCD serving as a horizontal charge transfer circuit. In FIG. 9, reference numeral 1 designates infrared detectors such as PtSi/Si Schottky barrier diodes. Reference numeral 2 designates vertical CSDs for transferring signal charges and reference numeral 3 designates a CSD scanner for driving the vertical CSD 2. Reference numeral 4 designates transfer gates (TG) for controlling the charge transfer from the infrared detector 1 to the vertical CSD 2 and reference numeral 5 designates a TG scanner for driving the transfer gates 4. Reference numeral 6 designates bus lines connecting the transfer gates 4 with the TG scanner 5. Reference numeral 7 designates a horizontal CCD for transferring signal charges and reference numeral 8 designates a CCD scanner for driving the CCD 7. Reference numeral 9 designates an output amplifier.
Operation thereof will be described. Infared rays radiated from the subject are incident on the photodetectors 1 arranged in a two-dimensional array and then converted into electricity in the photodetector 1. The signal charges thus generated are transferred to the vertical CSD 2 by opening the transfer gate 4. The switching of the transfer gate 4 is controlled by the TG scanner 5 connected to the transfer gate 4 by the bus line 6. When, the CSD scanner circuit 3 is driven, the signal charges in the vertical CSD 2 are transferred downward in the CSD 2 to reach the horizontal CCD 7. When the CCD scanner 8 is operated, the signal charges in the horizontal CCD 7 are transferred in the right direction in the CCD 7 to be output through the output amplifier 9. Then, signals from the photodetectors 1 arranged in a two-dimensional array are successively read out, whereby the intensity distribution of the infrared rays incident on the element is displayed on the monitor as an infrared image.
FIG. 10 shows a structure of a conventional infrared solid state imaging element having Schottky barrier diodes as photodetectors in which the signal charges are read out by an MOS system. In FIG. 10, reference numeral 1 designates infrared detectors such as PtSi/Si Schottky barrier diodes. Reference numeral 10 designates vertical MOS transistors for reading out signal charges and reference numeral 11 designates a vertical scanner for controlling the switching of the vertical MOS transistors 10. Reference numeral 12 designates bus lines connecting the vertical MOS transistors 10 with the vertical scanner 11. Reference numeral 13 designates a horizontal MOS transistor for reading out signal charges and reference numeral 14 designates a horizontal scanner for controlling the switching of the horizontal MOS transistors 13. Reference numeral 15 designates bus lines connecting the horizontal MOS transistors 13 with the horizontal scanner 14. Reference numeral 9 designates an output amplifier.
Operation thereof will be described. Infared rays irradiated from the subject are incident on the photodetectors 1 arranged in a two-dimensional array and then converted into electricity in the photodetectors 1 similarly as in FIG. 9. The signal charges thus generated are read out by the MOS system. More specifically, the signal charges from the photodetector 1 provided where a bus line 12 in a transverse direction selected by the vertical scanner 11 intersects a bus line 15 in a longitudinal direction selected by the horizontal scanner 14 are output through the output amplifier 9. The signal charges from the photodetectors 1 arranged in a two-dimensional array are successively read out and then the intensity distribution of the infrared rays incident to the element are displayed on the monitor as an infrared image.
FIG. 11 shows a structure of a conventional infrared solid-state imaging element having Schottky barrier diodes as photodetectors and CCDs as vertical and horizontal charge transfer circuits. In FIG. 11, reference numeral 1 designates infrared detectors such as PtSi/Si Schottky barrier diodes. Reference numeral 13 designates vertical CCDs for transferring signal charges and reference numeral 14 designates a CCD scanner for driving the CCDs. Reference numeral 4 designates transfer gates (TG) for controlling the charge transfer from the infrared detectors 1 to the vertical CCD 13. Reference numeral 15 designates an input pin for inputting a clock signal for driving the transfer gates 4. Reference numeral 6 designates bus lines connecting the input pin 15 with the transfer gates 4. Reference numeral 7 designates a horizontal CCD for transferring signal charges and reference numeral 8 designates a CCD scanner for driving the horizontal CCD 7. Reference numeral 9 designates an output amplifier.
In this infrared imaging element, unlike the infrared imaging element shown in FIG. 9, a CCD is used for the charge transfer in vertical direction. The operation thereof is fundamentally the same as that of the element shown in FIG. 9 except that the switching of the transfer gate 4 is controlled by the clock signal applied to the input pin 15 and the vertical CCDs 13 are controlled by the CCD scanner 14.
The infrared solid-state imaging elements shown in FIGS. 9, 10 and 11 are formed by a silicon LSI process. During the process, breakage of Al wirings for the bus lines 6, 12 and 15 may occur.
When the bus line 6 is broken in the infrared solid state elements shown in FIGS. 9 and 11, the transfer gate 4 on the right of the broken bus line in the figure cannot be opened and the signal charges from the photodetectors 1 cannot be read out. As a result, in the solid-state imaging element including such a broken bus line, an image defect A having continuous insensitive portions in the transverse direction as shown in FIG. 15 appears on the output image.
When the bus lines 12 and 15 are broken in the infrared solid-state imaging element shown in FIG. 10, an image defect A or B having continuous insensitive portions in the transverse direction or the longitudinal direction appears on the output image. In addition, when a diode is faulty or a contact part of the transfer gate is open, a black spot defect C as shown in FIG. 15 appears.
As a method for detecting such defects, the output image of an assembled is detected. This causes elements including defects to pass through a wafer test process or an assembly process, so that much time and a high cost are unfavorably incurred.
As another method for detecting these defects, the element may be driven in a wafer test. In a case of an infrared imaging element using Schottky barrier diodes, it is necessary to cool the element down to approximately 77K to operate the detector. However, it is technically difficult to perform a wafer test at such a low temperature.